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From:   <wuyun.wu@huawei.com>
To:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        "David Rientjes" <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        "Andrew Morton" <akpm@linux-foundation.org>
CC:     <hewenliang4@huawei.com>, <hushiyuan@huawei.com>,
        Abel Wu <wuyun.wu@huawei.com>,
        "open list:SLAB ALLOCATOR" <linux-mm@kvack.org>,
        "open list" <linux-kernel@vger.kernel.org>
Subject: [PATCH] mm/slub: branch optimization in free slowpath
Date:   Thu, 13 Aug 2020 18:18:08 +0800
Message-ID: <20200813101812.1617-1-wuyun.wu@huawei.com>
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From: Abel Wu <wuyun.wu@huawei.com>

The two conditions are mutually exclusive and gcc compiler will
optimise this into if-else-like pattern. Given that the majority
of free_slowpath is free_frozen, let's provide some hint to the
compilers.

Tests (perf bench sched messaging -g 20 -l 400000, executed 10x
after reboot) are done and the summarized result:

	un-patched	patched
max.	192.316		189.851
min.	187.267		186.252
avg.	189.154		188.086
stdev.	1.37		0.99

Signed-off-by: Abel Wu <wuyun.wu@huawei.com>
---
 mm/slub.c | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 4f496ae5a820..f9182a760675 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2958,20 +2958,21 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
 
 	if (likely(!n)) {
 
-		/*
-		 * If we just froze the page then put it onto the
-		 * per cpu partial list.
-		 */
-		if (new.frozen && !was_frozen) {
+		if (likely(was_frozen)) {
+			/*
+			 * The list lock was not taken therefore no list
+			 * activity can be necessary.
+			 */
+			stat(s, FREE_FROZEN);
+		} else if (new.frozen) {
+			/*
+			 * If we just froze the page then put it onto the
+			 * per cpu partial list.
+			 */
 			put_cpu_partial(s, page, 1);
 			stat(s, CPU_PARTIAL_FREE);
 		}
-		/*
-		 * The list lock was not taken therefore no list
-		 * activity can be necessary.
-		 */
-		if (was_frozen)
-			stat(s, FREE_FROZEN);
+
 		return;
 	}
 
-- 
2.28.0.windows.1